1. Field of the Invention
The invention is, in one instance, related to a production method of GaN or other group III-nitride crystals for use as nutrient or seed crystals in the ammonothermal method. GaN crystals grown using the current invention can also be used for successive device fabrication.
2. Description of the Existing Technology
(Note: This patent application refers to several publications and patents as indicated with numbers within brackets, e.g., [x]. A list of these publications and patents can be found in the section entitled “References.”)
Gallium nitride (GaN) and its related group III alloys are the key material for various opto-electronic and electronic devices such as light emitting diodes (LEDs), laser diodes (LDs), microwave power transistors, and solar-blind photo detectors. Currently LEDs are widely used in cell phones, indicators, displays, and LDs are used in data storage disc drives. The majority of these devices are grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide. The heteroepitaxial growth of group III nitride causes highly defected or even cracked films, which hinders the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting or high-power microwave transistors.
Most of the problems inherent in heteroepitaxial growth could be avoided by instead using homoepitaxial growth. Single crystalline group III nitride wafers can be sliced from bulk group III nitride crystal ingots and then utilized for high-end homoepitaxial growth of optical and electronic devices. For the majority of devices, single crystalline GaN wafers are desired because it is relatively easy to control the conductivity of the wafer, and GaN wafers will provide the smallest lattice/thermal mismatch with device layers. However, the GaN wafers needed for homoepitaxial growth are currently expensive compared to heteroepitaxial substrates. This is because GaN wafers are currently produced with quasi-bulk growth method in which a thick layer of GaN is grown with hydride vapor phase epitaxy (HVPE) on a heteroepitaxial substrate followed by removal of the substrate. Due to an open reactor configuration for HVPE, the growth efficiency is not as high as the conventional bulk growth method used for other semiconductor materials such as Si and GaAs.
Although a “real” growth method of bulk GaN is ideal, it has been difficult to grow group III nitride crystal ingots due to their high melting point and high nitrogen vapor pressure at high temperature. Growth methods using molten Ga, such as high-pressure high-temperature synthesis [1,2] and sodium flux [3,4], have been proposed to grow GaN crystals. Nevertheless the crystal shape grown using molten Ga is a thin platelet because molten Ga has low solubility of nitrogen and a low diffusion coefficient of nitrogen.
The ammonothermal method, which is a solution growth method, is a promising alternative for bulk GaN growth and has been demonstrated to grow real bulk GaN ingots [5]. High-pressure ammonia, which has high transport speed and high solubility of GaN, is used as a fluid medium to grow bulk GaN. State-of-the-art ammonothermal method [6-8] requires a sufficient supply of source material. While pure Ga metal can be used as a source material, it provides an uneven growth rate as the surface of the Ga nitridizes. To provide a more stable growth rate, polycrystalline GaN is desirable as a starting material.
One method to produce GaN polycrystals is direct nitridization of Ga with ammonia [9]. Nevertheless, this method can only yield powder form of GaN (i.e. microcrystalline or nanocrystalline).
On the other hand, HVPE which utilizes gaseous ammonia, gaseous hydrogen chloride and metallic Ga is commonly used to produce GaN wafers for successive device fabrication. We found that HVPE can be applied to produce source materials for the ammonothermal method.
With HVPE, single crystalline GaN seed can be grown on a main susceptor and parasitic polycrystalline GaN deposited inside the reactor can also be used as a nutrient for the ammonothermal growth. However, the current HVPE reactors are designed to grow GaN for relatively short duration, typically for between 1 and 10 hours. In order to apply HVPE to produce source materials for the ammonothermal method, the reactor must be modified to extend growth duration.